Presentation on theme: "THERMOREGULATION Peri-operative Teaching June 2008 Dr Mohua Jain Specialist Anaesthetist."— Presentation transcript:
THERMOREGULATION Peri-operative Teaching June 2008 Dr Mohua Jain Specialist Anaesthetist
Definitions CORE TEMPERATURE PERIPHERAL TEMPERATURE NORMOTHERMIA HYPOTHERMIA
CORE TEMPERATURE Thermal compartment of body, highly perfused tissues, uniform and higher temperature. Trunk, brain – 2/3 body heat PERIPHERAL TEMPERATURE Skin, subcutaneous – all body, inc limbs Usually 2 to 3 °C below core but can be much more
Core and peripheral temperatures both influence comfort about equally. Only core influences metabolic processes As peripheral temp drops, heat flows from core to periphery (gradient)
NORMOTHERMIA Core temp range of 36°C to 38°C HYPOTHERMIA Core temp less than 36°C MILD HYPOTHERMIA Core temp range 34°C to 36°C
Definitions (NICE) Preoperative - 1 hour before induction Intraoperative - the total anaesthesia time Postoperative - 24 hours after entry into the recovery area in the theatre suite Hypothermia - a patient core temperature of below 36.0°C. Comfortably warm - the expected normal temperature range of adult patients Temperature - used to denote core temperature
Maintenance of Heat Balance of heat production and loss Nervous system Hormones Vessels Behaviour Variations during day and month
Heat Production Metabolism Exercise Shivering Non-shivering thermogenesis (fat and muscle) Basal metabolic rate (BMR) is energy needed to maintain constant temperature
Heat Loss Radiation (40 to 60%) Convection (25 to 30%) Evaporation (10 to 20%) Respiration (10% by heating of air and evaporation) Energy loss can be up to 15 x BMR Sweating can be up to 1 litre per hour for short time, taking heat with it!
Don’t forget Hypothermia can be present regardless of temperature if patient complains of feeling cold or has the obvious signs Body needs to maintain set temperature as all processes involving enzymes are sensitive to temp and pH
Signs Usually below 36.5°C Peripheral vasoconstriction (esp stressed patients) Hairs standing on end (pilo-erection) Shivering Cold peripheries High diastolic blood pressure Importance of ‘behavioural’ actions
Measurement of Core Temperature accurate (patient, operator, instrument - variable readings) consistent, repeatable, keeping up with rapid changes, accessible, safe
Adverse Effects of Hypothermia CNS (Nervous system) RS (Respiratory system) CVS (Cardiovascular system) Renal and electrolytes Immune Blood Drug effects Others
CNS Reduced neuronal function Confusion Disorientation Stupor Raised intracranial pressure from shivering Seizures Coma
RS Hyperventilation then hypoventilation Lower respiratory rate Lower volumes (effect on CNS) Increased oxygen consumption from shivering Organ ischaemia
CVS More adrenaline (and other catecholamines) Vasoconstriction Raised blood pressure Bradycardia Myocardial ischaemia and infarction ECG changes Arrhythmias
Renal & Electrolytes ‘Cold diuresis’ Renal tubule damage Constriction of skin and gut vessels Potassium, Magnesium, Calcium and Phosphate all decrease
Immune Infections Wound breakdown and infections Collagen linking less as oxygen drops Less subcutaneous oxygen White blood cells function less
Blood Less coagulation Less platelet function More viscosity More blood loss More blood transfusions
Drug effects Usually prolongs actions of all drugs, (esp those needing enzymes for their metabolism) Muscle relaxants and opiates last longer Less IV and volatile agents needed for same degree of unconsciousness
Other More patient discomfort with shivering More time in PACU / Recovery Thresholds for pain and nausea Difficulty with cannulation More time in hospital More time to establish diet More costs from all above
Shivering Usually temperature related – uncomfortable involuntary rhythmic muscle contractions to maintain core temperature Complex, patterns of tremors Can occur post GA or during labour even with normal temperature. Mechanism unknown ?pain and stress Post-op 20 to 40%? Problem for monitoring Elderly rarely shiver
Effects of General and Regional Anaesthesia Impaired thresholds for responses so they happen later 3 stage drop in temperature 1 to 3°C 1.Rapid in 1 st hour (Redistribution of heat from core to periphery - vasodilation) 2.Gradual (Heat loss causes then exceed heat production causes) 3.Plateau (Production catches up)
So far... Definitions Heat balance – how and why needed Measurement of core temperature Bad effects of Hypothermia Shivering (normally and post-op) Anaesthesia So, how can we prevent hypothermia?
Evidence - Research and Clinical Recommendations and guidelines (esp 2000 onwards) WHO - ambient temperature American Society of Anesthesiologists (ASA) American Society of PeriAnesthesia Nurses (ASPAN) National Institute of Clinical Excellence (NICE)
Common Sense Guidelines Minimising heat loss from the body Giving heat to the body
Common Sense Guidelines Pre-operative Intra-operative Post-operative ASSESSMENT (identify, measure, observe & ask) INTERVENTION (preventative, passive and active)
Identification of Risks Very young Very old Female GA / RA Large surface area / gut exposed Ambient temp (circulating air) Poor nutritional status Length of surgery Fluid shifts Irrigation fluids Trauma/burns Cold transfers
Patients at higher risk of perioperative hypothermia (NICE) Some patients are at higher risk of inadvertent perioperative hypothermia; they should be managed accordingly if any two of the following apply: ASA grade II to V preoperative temperature below 36.0°C undergoing combined general and regional anaesthesia undergoing major or intermediate surgery at risk of cardiovascular complications.
Expectations Core temperature never to drop below 36°C at any stage To avoid symptoms and signs If GA will last 30 mins or more, must measure temp through operation More strict if high risk group Start actions BEFORE theatre
Preoperative warming If the patient’s temperature is below 36.0°C in the hour before they leave the ward or emergency department: forced air warming should be started preoperatively on the ward or in the emergency department (unless there is a need to expedite surgery because of clinical urgency) forced air warming should be maintained throughout the intraoperative phase.
Intraoperative phase The patient’s temperature should be measured and documented before induction of anaesthesia and then every 30 minutes until the end of surgery. Induction of anaesthesia should not begin unless the patient’s temperature is 36.0°C or above.
Passive – to minimise heat loss For hypothermic AND normothermic patients Ambient temp at least 20°C (upto 30° if burns or neonates!) Passive insulation (layer of air) Warmed cotton aircell blankets Space blanket? Circulating water mattress? Hats (esp Paeds) Socks etc (Special cases – pre veins, post flaps) (Before – preop vasodilation)
Active – add to heat gain For hypothermic patients Skin – Forced air warming / convective (Bair Hugger) – upto 50 W heat given (no infection evidence) Internal – IV, irrigation (1 litre fluid at room temp will lower core temp by 0.25°C) Airway - humidification (HMEF)
Cardiopulmonary bypass Dialysis (Protein infusion to increase metabolism) Watch out for over-heating of skin and fluids (keep below 45°C)
Warming intravenous fluids Intravenous fluids (500 ml or more) and blood products should be warmed to 37°C using a fluid warming device.
Postoperative phase The patient’s temperature should be measured and documented on admission to the recovery room and then every 15 minutes Ward transfer should not be arranged unless the patient’s temperature is 36.0°C or above. If the patient’s temperature is below 36.0°C, they should be actively warmed using forced air warming until they are discharged from the recovery room or until they are comfortably warm
Costs and savings per 100,000 population Recommendations with significant costs Costs (£ per year) Increased use of forced air warming blankets43,000 Increased warming of IV fluids and blood products 23,000 Estimated cost of implementation66,000 Recommendations with significant savings Savings (£ per year) Expected reduction in surgical site infections–43,000 Estimated annual net cost of implementation23,000
Discussion Which key areas of local practice differ from the guideline? To ensure effective implementation: -what equipment is needed? -what are staff training needs? What will the impact be on the average length of patient stay if the guideline is implemented fully? How should Risk and Safety Managers be involved in the implementation of the guideline?
SUMMARY Understanding of heat balance Understanding why this is important Why to prevent temp below 36°C How to measure temperature Recommendations of how to assess Passive and active ways of helping the patient from pre- to post-op